1. Field of the Invention
The present invention relates to a display driver, and particularly to a display driver which can be appropriately used in a display driver Integrated Circuit (IC) with a built-in image memory.
2. Background Art
In a display driver, on which a Static Random Access Memory (SRAM) that is an image memory is mounted, for example, a Liquid Crystal Display (LCD) driver, and in a product to which a process rule using a process technology equal to or greater than 130 nm is applied, an off-leakage of a Metal Oxide Semiconductor (MOS) transistor is small enough, and a leakage current flowing through the whole SRAM is small enough compared to operation power of the LCD driver, and thus influence on a consumption current of the whole LCD driver is small to a negligible degree. Recently, as the number of pixels of the LCD panel is increased, memory capacity of the SRAM mounted on the LCD driver is increased in class by several tens of Mbit, and process shrinking is underway for chip size reduction. For example, in case that a process moves from a 130 nm process to a 90 nm process, a power supply voltage is lowered from 1.5 V to 1.3 V in relation to a breakdown voltage of the MOS transistor, but at the same time, the MOS transistor performance is maintained, and thus a threshold voltage (Vth) is also required to decrease. As a result, a problem happens that the off-leakage current of the SRAM increases and the consumption current of the whole LCD driver increases to a degree that cannot be ignored.
In JP-A-2008-191442, a technology is disclosed which decreases the off-leakage current of a memory mounted on the display driver IC and makes stable a normal operation of the memory. Depending on which operation mode of a normal operation mode and a stand-by mode is selected for the display driver IC, an ON and OFF control of a switch transistor connected to a power supply of the memory is performed, power supply delivery to an unnecessary portion is cut off, and the off-leakage current is reduced. In the switch transistor, the power supply of a voltage which is higher to a degree where a voltage drop generated by the switch transistor can be compensated for, is connected thereto, and thereby the power supply of the memory itself can be maintained to a high value, and the normal operation of the memory can become stable.